SECRETION OF SHIGELLA-FLEXNERI IPA INVASINS ON CONTACT WITH EPITHELIAL-CELLS AND SUBSEQUENT ENTRY OF THE BACTERIUM INTO CELLS ARE GROWTH STAGE-DEPENDENT

Upon contact with the surface of epithelial cells, Shigella flexneri s ecretes Ipa proteins through the Mxi-Spa type In secretion apparatus. Among the Ipa proteins, IpaB and IpaC form a soluble complex in the ba cterial supernatant which appears to be sufficient to initiate the cel lular rearrangements necessary to achieve bacterial entry. Here, we pr ovide the first evidence that efficiency of bacterial entry into cells depends on the stage of bacterial growth. Bacteria in the early phase of exponential growth are six times more invasive than those in the s tationary phase. The entry efficiency of the bacteria present on the c ell surface appears to correlate with the percentage of those that are able to secrete their invasins. This suggests that the capacity to ac tivate the Mxi-Spa apparatus is a major factor in the regulation of ba cterial entry efficiency. Consistent with these observations, we have further shown that bacteria which have reached the stage of division s ecrete Ipa proteins more often than those that have not. Also, initial secretion occurs essentially in the area of the septation furrow. The Ipa proteins, secreted in the vicinity of the septation furrow, seem to initiate the early stages of reorganization of the host cell cytosk eleton.